The present invention relates generally to three-dimensional (3D) computerized tomography (CT) and, more particularly, to methods and systems for minimizing the amount of missing data and, at the same time, avoiding corrupted data and resulting artifacts in image reconstruction when a cone beam configuration is used to image a portion of an object which is larger than the field of view.
In conventional computerized tomography for both medical and industrial applications, an x-ray fan beam and a linear array detector are employed. Two-dimensional (2D) imaging is achieved. While the data set is complete and image quality is correspondingly high, only a single slice of an object is imaged at a time. When a 3D image is required, a "stack of slices" approach is employed. Acquiring a 3D data set one 2D slice at a time is inherently slow. Moreover, in medical applications, motion artifacts occur because adjacent slices are not imaged simultaneously. Also, dose utilization is less than optimal, because the distance between slices is typically less than the x-ray collimator aperture, resulting in double exposure to many parts of the body.
One approach to acquiring a 3D data set simultaneously is described in the literature: Richard A. Robb, Arnold H. Lent, Barry K. Gilbert, and Aloysius Chu, "The Dynamic Spatial Reconstructor", J. Med. Syst., Vol. 4, No. 2, pp. 253-288 (1980). The Dynamic Spatial Reconstructor employs twenty-eight x-ray sources and twenty-eight x-ray imaging systems in a synchronous scanning system to acquire data for a conventional "stack of slices" reconstruction all at once. The actual geometry is a stack of twenty-eight cone beams scanning twenty-eight respective cylindrical volumes, with area detectors employed to acquire 240 adjacent video lines of data for each slice. However, the data is analyzed as though it is from a stack of fan beam projections, stacked in an axial direction, using conventional 2D reconstruction algorithms. Consistent with this approach, in the Dynamic Spatial Reconstructor the divergence of the x-ray beam above and below the central slice of each cylindrical volume is only .+-.4.degree..
In a system employing true cone beam geometry, a cone beam x-ray source and a 2D area detector are employed. An object is scanned, preferably over a 360.degree. angular range, either by moving the x-ray source in a scanning circle around the object, while keeping the 2D area detector fixed with reference to the source, or by rotating the object while the source and detector remain stationary. In either case, it is relative movement between the source and object which effects scanning. Compared to the conventional 2D "stack of slices" approach to achieve 3D imaging the cone beam geometry has the potential to achieve rapid 3D imaging of both medical and industrial objects, with improved dose utilization.
The cone beam geometry for 3D imaging has been discussed extensively in the literature, as represented by the following: M. Schlindwein, "Interactive three-Dimensional Reconstruction from Twin-Cone Beam Projections", IEEE Trans. Nucl.Sci., Vol. NS-25, No. 5, pp. 1135-1143 (October 1978); Gerald N. Minerbo, "Convolutional Reconstruction from Cone-Beam Projection Data", IEEE Trans. Nucl. Sci., Vol. NS-26, No. 2, pp. 2682-2684 (April 1979); Heang K. Tuy, "An Inversion Formula for Cone-Beam Reconstruction", SIAM J. Math., Vol. 43, No. 3, pp. 546-552 (June 1983); L. A. Feldkamp, L. C. Davis, and J. W. Kress, "Practical Cone-Beam Algorithm", J. Opt. Soc. Am. A., Vol. 1, No. 6, pp. 612-619 (June 1984); Bruce D. Smith, "Image Reconstruction from Cone-Beam Projections: Necessary and Sufficient Conditions and Reconstruction Methods", IEEE Trans. Med. Imag., Vol. MI-44, pp. 14-25 (March 1985); and Hui Hu, Robert A. Kruger, and Grant T. Gullberg, "Quantitative Cone-Beam Construction", SPIE Medical Imaging III: Image Processing, Vol. 1092, pp. 492-501 (1989).
A typical scanning and data acquisition configuration employing cone-beam geometry is depicted in FIG. 1. An object 20 is positioned within a field of view between a cone beam x-ray point source 22 and a 2D detector array 24, which provides cone beam projection data. An axis of rotation 26 passes through the field of view and object 20. For purposes of analysis, a midplane 28 is defined which contains the x-ray point source 22 and is perpendicular to the axis of rotation 26. By convention, the axis of rotation 26 is referred to as the z-axis, and the intersection of the axis of rotation 26 and the midplane 28 is taken as the origin of coordinates. x and y axes lie in the midplane 28 as indicated, and the (x,y,z) coordinate system rotates with the source 22 and detector 24. For scanning the object 20 at a plurality of angular positions, the source 22 moves relative to the object 20 and the field of view along a circular scanning trajectory 30 lying in the midplane 28, while the detector 24 remains fixed with respect to the source 22.
Thus, in the configuration of FIG. 1, data are acquired at a number of angular positions around the object by scanning the source and detector along the single circular scanning trajectory 30 (or equivalently rotating the object while the source and detector remain stationary).
However, there are two limitations of the prior art single circular scanning trajectory approach which are addressed by the present invention. The first limitation is that the data set acquired is incomplete for 3D image reconstruction. The second limitation occurs when the field of view to be imaged is smaller than the entire object, a common situation in both industrial and medical applications. In this situation, some measured projections contain information from both the field of view of interest and from other regions of the object outside the field of view. The image of the field of view of interest is therefore corrupted by data resulting from overlying material.
Regarding the incomplete data set limitation, Smith, 1985, above, has shown that a cone beam data set is complete if there is a point from the x-ray source scanning trajectory on each plane passing through the object of interest (with the assumptions that the detector is locked in position relative to the source and large enough to span the object under inspection). A configuration suggested by Minerbo (1979, above) and Tuy (1983, above), which Smith points out satisfies his condition for data completeness, is to employ two circular source scanning trajectories which are perpendicular to each other. Such a scanning configuration is however difficult to implement as a practical matter. Moreover, it presumes that the object fits within the scanning trajectories, and does not address the situation where an object is larger than the field of view.